Following in-service incidents of aircraft fuel tank explosions, new regulations to achieve flammability reduction in fuel tanks have been introduced for new commercial aircraft.
The relevant FAA Regulation states: “A fuel tank is considered inert when the bulk average oxygen concentration within each compartment of the tank is 12 percent or less from sea level up to 10,000 feet altitude, then linearly increasing from 12 percent at 10,000 feet to 14.5 percent at 40,000 feet altitude, and extrapolated linearly above that altitude.”
Typically this is achieved by continuously adding Nitrogen to the fuel tank ullage during operation and keeping the Ullage Gas Oxygen Fraction (UGOF) below 11%. Currently, one solution is to use Air Separation Modules (ASMs) to molecularly separate the Nitrogen from the Oxygen in air, either obtained as engine bleed air or compressed external air.
On future aircraft it is proposed to use fuel cell technology to provide the auxiliary power for such functions as ground operations, ground propulsion and emergency in-flight power. As a by-product of this technology where Hydrogen is catalytically reacted with Oxygen (supplied from the ambient air) to produce electric power, Oxygen Depleted Air (ODA) is produced where the Oxygen concentration is typically <10.5%. This air is also very humid (approximately 100%) and hot, so it needs to be processed to reduce both its humidity and temperature before it can be used to inert the fuel tanks.
Reducing the temperature and humidity is a known technology however it consumes aircraft resources, in particular creating a high demand when other systems are also at a high demand, typically on aircraft descent into hot destinations.